In the field of wall covering board materials, to afford comforts and/or decorative effects to the interior of cars, ships or buildings, a corrugated paperboard has recently attracted increasing attention as a basic material because of its light weight, low cost, good heat and sound insulating ability and fairly good adaptability to various mechanical processing. In automobiles, for example, trim boards such as roof trim boards of corrugated paperboard base have already been in practical use on an industrial scale.
The trim boards of this type usually consist of a substrate of shaped corrugated paperboard and a skin or facing layer which is laid on one side of the substrate for producing protective, decorative and/or cushioning effects. In most cases, the shaped corrugated paperboard takes the form of a double-faced board given by bonding liners to both sides of a corrugated medium using a thermoplastic resin as the adhesive. A variety of sheet materials are usable as the facing layer. The useful materials may roughly be classified into woven or non-woven cloths, soft plastics sheets, artificial leathers and laminated materials given by any of these sheet materials with a cushioning layer such as a urethane foam layer. Since walls (including ceiling) to be covered with these trim boards usually have curved surfaces, the trim boards are produced as at least partly curved boards in conformance with the curved wall surfaces. Press-forming with application of heat is usually used for producing curved trim boards of corrugated paperboard. In most cases, a flat or press-formable corrugated paperboard) alone is formed into a desired shape by means of a hot-press, followed by the application of the facing layer onto the shaped corrugated paperboard with the interposal of a hot-melt type adhesive layer and followed by the application of another hot-press to accomplish the bonding between the shaped paperboard and the facing layer.
However, the above-mentioned method has encountered the following problem at the time of press-forming of the corrugated paperboard substrate, originating from the tight bonding between each side of the corrugated medium and the bonding liner. When the press-formable substrate is press-formed into a partly or entirely curved board, wrinkles tend to appear in curved areas of the liner on a smaller radius side which undergoes a remarkable contraction to give shrinkages. This will be well understood from FIGS. 1A and 1B.
Usually, the afore-mentioned wrinkles are covered by the facing layer. However, the presence of wrinkles in the shaped paperboard frequently results in that the outer surface of the facing layer shows visible traces of the wrinkles since the facing layer is quite thin. The presence of such traces of course impairs the decorative effect and hence the commercial value of the trim board.